The present invention relates to a lid lock for washing machines and more generally to a memory wire actuator useful for this and other applications.
During the spin cycle of a washing machine, water is removed from wet clothes centrifugally by spinning the clothes at high speed in a spin basket. In order to reduce the possibility of injury to the user, the washing machine lid, blocking access to the spin basket, may be locked in the closed position during the wash and spin cycle. Lid locks for this purpose are often designed to hold the lid closed for a period of time after the conclusion of the spin cycle or a power failure to allow the spinning basket time to halt.
This “time delay” feature can be provided in a lid lock by using a thermal element, such as a wax motor or a bimetallic strip, both of which can be heated electrically to actuate the lock, and then which must cool for a period of time after heating stops before it releases. Wax motors employ a piston and cylinder, the latter which contains wax or other expanding material. When the wax is heated, the piston is ejected from the cylinder to perform the necessary locking feature. As the cylinder cools, the piston is retracted under spring force. Heat may be applied to the wax motor through the use of a positive temperature coefficient (PTC) resistor glued to one face of the wax motor casing.
Bimetallic strips typically employ two dissimilar metals (e.g., having different coefficients of thermal expansion) laminated together so that heating of the strip causes the strip to curve and lock the lid. Bimetallic strips may be heated by passing electrical current directly through the metal of the strip, an approach that is not practical with wax motors, or by a PTC resistor attached to a portion of a flat side of the bimetallic strip.
Memory shape alloys are a class of alloys that can recover from strain when they are heated to a particular temperature. A wire form of memory shape alloy, known as “muscle wire”, contracts several percent of its length when heated and can be used to provide a thermal actuator. Muscle wire is normally heated by passing a low current through its length. This process heats the entire length of the wire, maximizing its actuation length change, and yet does not interfere with the wire's expansion. Direct heating of the muscle wire also provides for fast response.
A significant obstacle to using muscle wire in commercial applications, such as in a lid lock for a washing machine, is the need to precondition the high voltage line power normally received by an appliance to a low voltage that may be applied to the muscle wire, and to regulate that voltage to eliminate overheating of the muscle wire that can damage its strain recovery properties.